Roller-type wave generators are known as wave generators for wave gear devices. A roller-type wave generator is provided with a plurality of rollers to flex a flexible externally toothed gear of a wave gear device into an ellipsoidal shape. Patent documents 1 and 2 disclose a two-roller-type wave generator for flexing a cup-shaped flexible externally toothed gear into an ellipsoidal shape using two rollers. Patent document 3 discloses a two-roller-type wave generator, and also discloses a six-roller-type wave generator provided with a pair of rollers arranged on a major axis in a cup-shaped flexible externally toothed gear flexed into an ellipsoidal shape; and two pairs of rollers arranged between the major axis and a minor axis.